This invention relates to a steerable drilling system and components thereof for use in the formation of, for example, a wellbore for use in the extraction of hydrocarbons.
A known steerable drilling system comprises a downhole motor used to drive a drill bit for rotation about an axis thereof. A bias unit is located between the motor and the drill bit and arranged to apply a biasing, sideways acting load to the drill bit to urge the drill bit form a curve in the borehole being drilled. The bias unit typically comprises a housing upon which a number of movable, for example pivotable, flaps or pads are mounted, and actuators in the form of pistons associated with the pads to drive the pads between retracted and extended positions. A control unit is provided to control the operation of the actuators. The control unit may include a valve arrangement for controlling the application of pressurised fluid to the pistons, and hence to control the position adopted by the pads at any given time. By appropriate control, the pads can be urged against one side of the wall of the bore being formed to apply a side load to the bias unit and any component secured thereto, for example the drill bit, thereby allowing the drill bit to be steered.
In use, when a curve, or dogleg, is to be formed in the wellbore, the control unit causes the actuators to move the pads between their retracted and extended positions as the bias unit rotates so that the pads apply a lateral or sideways acting biasing load to the bias unit and drill bit, the biasing load acting in a substantially constant direction causing the bit to form the desired dogleg in the wellbore.
As the bias unit operates by applying relatively high pressure fluid to one end of each piston, the other end having lower pressure fluid applied thereto, a significant fluid pressure drop must be present in the downhole environment in order for the fluid to operate. Typically, the bias unit requires a pressure drop of around 700 psi to function correctly. In some applications, the pressure at which drilling fluid can be supplied is restricted and, where other downhole components also require a pressure drop to operate correctly or efficiently, it may be undesirable or impractical to use a bias unit of this type.
Drilling fluid or mud powered motors, for example in the form of progressive cavity motors known as Moineau motors, are becoming increasingly commonly used in this type of application. However, the use of such motors in conjunction with bias units of the type mentioned hereinbefore is problematic as the control unit for the bias unit is located between the motor and the bias unit resulting in these components being spaced apart from one another by a significant distance. This can limit achievable build and turn rates. Further, where the control unit controls the supply of fluid under pressure to the actuators, the fluid must be supplied through or past the motor.